RE: [RFC] raw_copy_from_user() semantics

[David Laight]

From: Catalin Marinas
> Sent: 22 July 2020 12:37
> 
> On Sun, Jul 19, 2020 at 12:34:11PM -0700, Linus Torvalds wrote:
> > On Sun, Jul 19, 2020 at 12:28 PM Linus Torvalds
> > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > > I think we should try to get rid of the exact semantics.
> >
> > Side note: I think one of the historical reasons for the exact
> > semantics was that we used to do things like the mount option copying
> > with a "copy_from_user()" iirc.
> >
> > And that could take a fault at the end of the stack etc, because
> > "copy_mount_options()" is nasty and doesn't get a size, and just
> > copies "up to 4kB" of data.
> >
> > It's a mistake in the interface, but it is what it is. But we've
> > always handled the inexact count there anyway by originally doing byte
> > accesses, and at some point you optimized it to just look at where
> > page boundaries might be..
> 
> And we may have to change this again since, with arm64 MTE, the page
> boundary check is insufficient:
> 
> https://lore.kernel.org/linux-fsdevel/20200715170844.30064-25-catalin.marinas@xxxxxxx/
> 
> While currently the fault path is unlikely to trigger, with MTE in user
> space it's a lot more likely since the buffer (e.g. a string) is
> normally less than 4K and the adjacent addresses would have a different
> colour.
> 
> I looked (though briefly) into passing the copy_from_user() problem to
> filesystems that would presumably know better how much to copy. In most
> cases the options are string, so something like strncpy_from_user()
> would work. For mount options as binary blobs (IIUC btrfs) maybe the fs
> has a better way to figure out how much to copy.

What about changing the mount code to loop calling get_user()
to read aligned words until failure?
Mount is fairly uncommon and the extra cost is probably small compared
to the rest of doing a mount.

	David

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